GeneticsCell Division

Reference: Chapter 5

Principles of Cell Division Cell organization

Cell Nucleus Chromosomes Genes DNA

Types of Cell Division Mitosis

Occurs throughout life Functions

growth, development, repair Nuclear division in which chromosome number

stays constant

Meiosis Only occurs in reproductive organs Reduces chromosome number by half

Vocabulary Review Chromatin

Genetic material is long, thin strands dispersed throughout the nucleus in a tangled, fibrous mass.

Individual chromosomes all mixed together Chromatin condenses and separates into

individual chromosomes before mitosis begins. Chromosomes

condensed rod-shaped DNA molecules Only form during cell division

Vocabulary Review cont’d Diploid (2N) number

characteristic chromosome number, chromosomes in pairs

Found in somatic cells Haploid (N) number

half the diploid number, found in gametes

Mitosis – Overview DNA replication

produces duplicated chromosomes

Composed of 2 sister chromatids

Genetically identical held together by a

centromere Centromere divides Each chromatid

becomes a daughter chromosome

Mitosis - Overview Fig 5.3

Mitosis: Stages Prophase

Nuclear membrane disappears, centrosomes migrate, spindle fibers appear

Metaphase chromosomes line up at equator, associated with

spindle fibers Anaphase

centromeres divide, sister chromatids migrate to opposite poles, cytokinesis begins

Telophase nuclear membranes form, spindle disappears,

cytokinesis occurs

Meiosis- Overview 2 divisions, 4 daughter cells Cells are diploid at beginning of meiosis

Human body cells have 46 individual chromosomes

Chromosomes can be arranged into a set of 23 matched pairs called homologues Similar in shape, size, and have genes which deal

with the same traits.

Meiosis- Overview cont’d Meiosis I

Homologues line up side by side at equator-synapsis

When pairs separate, each daughter cell receives one member of the pair

Cells are now haploid

Meiosis- Overview cont’d Fig 5.9

Meiosis- Overview cont’d Meiosis II

No replication of DNA occurs in this division Centromeres divide and sister chromatids migrate

to opposite poles to become individual chromosomes

Each of the four daughter cells produced has the haploid chromosome number and each chromosome is composed of one chromatid

Meiosis I - Stages Prophase I

Synapsis occurs, nuclear membrane breaks down Homologues line up side by side and crossing over

occurs Crossing over-exchange of segments of DNA

between homologues Crossing over-exchange of segments of DNA

between homologues Independent assortment of chromosome pairs

Independent alignment Fig 5.11

Synapsis and crossing over Fig 5.10

Meiosis I – Stages cont’d Metaphase I

Homologous pairs line up at equator such that maternal or paternal member may be oriented toward either pole

Anaphase I Homologous chromosomes (each still consisting of

2 chromatids) undergo independent assortment into daughter cells

Telophase I Cytokinesis produces 2 daughter cells which are

haploid

Interkinesis-period between meiosis I and meiosis II

Meiosis I in animal cells

Fig 5.12

Meiosis II - Stages Prophase II

Cells have 1 member of each homologous pair Metaphase II

Chromosomes line up at the equator Anaphase II

Centromeres divide and daughter chromosomes migrate

Telophase II Nuclei form, cytokinesis

Meiosis II Fig 5.13

Meiosis in Humans Spermatogenesis

Occurs in seminiferous tubules of testes Begins at puberty and continues throughout life Each meiotic division produces 4 haploid cells Generates small cells, no organelles, only 1N

nucleus

Meiosis in Humans cont’d Oogenesis

Begins in vitro, primary oocytes present at birth At puberty hormones stimulate 1 primary oocyte

to complete the first meiotic division each month. Each meiotic division produces only 1 haploid egg

Meiosis in Humans Oogenesis: cont’d

Cell division is asymmetrical Majority of cell contents stay in one half, will eventually

develop into ovum, containing all necessary organelles for all future cells.

Smaller cell, polar body, degenerates and disappears. Second meiotic division not completed unless egg

is fertilized by sperm.

Oogenesis

Spermatogenesis and oogenesis Fig 5.16